This invention relates generally to acoustic wave devices, and, more particularly, to acoustic wave devices employed in delay lines, oscillators, filters, and related applications.
In recent years, much research has been performed in the development of acoustic wave devices, primarily for use in communications and radar systems. There is a trend in such systems for operation at increasingly higher carrier frequencies, principally because the spectrum at lower frequencies is becoming relatively congested, and also because the permissible bandwidth is greater at higher frequencies. Congestion of the frequency spectrum is further aggravated by the use of spread spectrum techniques, in which a normally narrowband information signal is spread over a relatively wide band of frequencies, to provide enhanced immunity to noise and interference. Piezoelectric crystals have provided the basis for devices such as oscillators, resonators and filters, operating at very high radio frequencies, but piezoelectric crystals used in conventional modes of operation cannot conveniently meet the demand for higher frequencies in modern systems.
It has been known for some time, of course, that certain crystalline materials have piezoelectric properties. In particular, there is what is sometimes referred to as the direct piezoelectric effect, wherein electrical charges appear on crystal surfaces upon the application of an external stress. There is also a converse piezoelectric effect, wherein the crystal exhibits strain or deformation when an electrical charge is applied by external means to faces of the crystal. These effects have been employed for many years in crystal oscillators and other devices in which bulk acoustic waves are transmitted through a crystal, typically between electrode plates located at opposite faces of the crystal. Use of bulk waves in this manner has provided crystal oscillators and filters with relatively good temperature stability, but with frequencies limited to approximately 200 megahertz (MHz), and more typically falling below 40 MHz. Consequently, higher frequencies cannot be obtained without the expense of additional components, such as frequency multipliers.
In recent years, piezoelectric crystals utilizing surface acoustic waves, rather than bulk acoustic waves, have been developed, with frequencies of operation between 10 MHz and approximately 2 gigahertz (GHz). These surface acoustic wave devices, or "SAW" devices, have a number of advantages over the older bulk acoustic wave devices. In addition to their higher frequency of operation, SAW devices can be designed to have a weighted frequency response. Moreover, they have an operating frequency that is independent of crystal dimensions, and they have a planar structure that is mechanically rugged and can be readily fabricated using conventional semiconductor metalization techniques.
SAW devices utilize interdigital transducers for converting electrical energy into acoustic or mechanical energy, and vice versa. Basically, these transducers comprise metalized layers formed on the crystal surface in finger-like configurations, like the teeth of a comb. The finger-like elements are usually arranged in two sets, with the fingers in the two sets extending in opposite directions, in an interleaved fashion, from respective elongated pads known as sum bars. When an electrical signal is applied to such a transducer, across the sum bars of the two sets of fingers, a surface acoustic wave, also known as a Rayleigh wave, is launched in a direction perpendicular to the transducer fingers. When the surface acoustic wave encounters a second, similarly structured transducer, it is transformed back into an electrical output signal. Typically, a transmitting or input transducer in such a device is excited by an oscillatory electrical signal, either in continuous-wave (CW) mode, or in a pulsed mode of operation.
The frequency of operation of a surface acoustic wave device depends largely on the size and geometry of the transducers. Although an electroacoustical transducer will convert an input electrical signal to an acoustic wave of the same frequency, the transducer has a high insertion loss at frequencies outside a band of frequencies determined by the transducer geometry. A SAW device transducer operates, in effect, like a bandpass filter, the center frequency of which is determined by the spacing between pairs of transducer fingers, and the pass-band width of which is controllable to some degree by the number of pairs of fingers in the transducer. Generally, a transducer with many pairs of fingers will have a narrow-band frequency response, while one with few pairs of fingers will have a wideband frequency response.
Since the spacing of the transducer fingers is directly proportional to the wavelength at operating frequencies, and since the velocity of the acoustic wave is equal to the product of its frequency and its wavelength, it follows that the operating frequency of an acoustic wave device using a particular interdigital transducer is dependent only on the velocity of the acoustic wave. Moreover, since wave velocities in SAW devices are limited by the properties of available crystals, extremely small wavelengths would be required to produce very high frequency devices. To obtain such small wavelengths, interdigital transducers having very small finger spacings must be constructed. However, the fabrication techniques usually used in the manufacture of transducers are those of photolithography, as employed in metalization processes in semiconductor manufacture, and one major problem in further reducing the size of interdigital transducers is that the resolution obtainable in these techniques is limited by the wavelength of the light employed in the photolithographic process.
Another problem with SAW devices is that bulk acoustic waves are often launched into the crystal at the same time as the surface acoustic waves. The bulk waves tend to cause spurious frequency responses that significantly affect the operational integrity of the devices. Furthermore, since the devices utilize a surface acoustic wave, they are sensitive to surface contamination, and sometimes a piezoelectric surface overlay material must be used to enhance the coupling efficiency with which energy is converted between the electrical and acoustic forms. SAW devices also have relatively poor temperature stability, i.e., their frequency of operation varies substantially with temperature. Temperature changes affect the elastic constants relating stress to strain in crystals, and therefore affect the velocity of wave propagation to some degree, depending on the crystal material, type of crystal cut, and direction of propagation. Temperature also affects the physical size of the crystal, and therefore affects finger spacings and propagation delay times. Although some piezoelectric materials, such as ST-but quartz, have good temperature stability properties, they are not good materials for the propagation of surface acoustic waves. SAW devices also have a long term aging problem, due principally to accumulation of surface contaminants and possibly due to gradual relaxation of surface stress conditions caused by surface imperfections.
By way of further background, it is of interest to note that piezoelectricity has been demonstrated in many different crystal materials, but only a few such materials are used in practical transducer design. In both the direct and converse piezoelectric effects, the mechanical strains and stresses are related to electrical parameters, such as charge, voltage and polarization, by constants of proportionality which are referred to as the piezoelectric constants. There are different constants for different directions in the crystal. Furthermore, the mechanical stresses and strains within a crystal are related to each other by elastic constants, there again being different constants for the different directions in the crystal. In the most general case, there are eighteen piezoelectric stress constants and eighteen piezoelectric strain constants, although not all eighteen constants of each type exist in every piezoelectric material. For example, in quartz, one of the most commonly used piezoelectric materials, only five constants have non-zero values, and, by utilizing properties of symmetry within the crystal, only two such constants need be determined in order to completely define the properties of the crystal.
An important parameter in the study of acoustic wave phenomena is the electromechanical coupling coefficient for various types of waves. The efficiency with which electrical energy is converted into acoustic or mechanical energy by an interdigital transducer is highly dependent upon the orientation of the transducer with respect to the crystallographic axes of the piezoelectric material. When the coupling coefficient is zero, it is virtually impossible to launch an acoustic wave into the material. High coupling coefficients, on the other hand, indicate relatively high coupling efficiencies. For example, the coupling coefficient for surface acoustic waves in some cuts of quartz varies from a maximum in one direction of propagation, to zero in another direction.
It has been known for some time that interdigital transducers are sometimes responsible for the propagation of bulk shear waves into the crystal material, as well as surface waves along the crystal surface. For the most part, researchers in the field have focussed on techniques for identifying and eliminating such bulk waves, since they have been regarded as a source of interference with surface waves. However, one group of researchers theorized that bulk shear waves could propagate just beneath the surface of a ceramic material, and another group recognized that such bulk shear waves could be utilized on their own, if the surface waves were appropriately suppressed, as by means of an absorbent surface layer.
It will be appreciated from the foregoing that there is still a significant need for acoustic wave devices capable of operation at higher frequencies than SAW devices, and having none of the inherent disadvantages of SAW devices. The present invention is directed to this end.